In recent years techniques have been developed for the manufacture of laminated boards for panels from plastic materials. Such boards or panels have been made in a wide range of sizes for a wide range of applications. It is known, for example, that panels for use in bulding construction may be fabricated from plastic material and may offer great advantage over the more commonplace wood panels. Such plastic panels for use in building construction and the like may be for decorative applications and therefore relatively thin, such as wall paneling, or the panels may be adapted for use as structural members where strength is important and may be relatively thick, such as a door. Another important application of laminated plastic boards is the construction of packaging cartons for storing or transporting materials such as food products or commercial and industrial articles of various kinds. This application of plastic board is, in some measure, supplanting the more conventional corrugated cardboard made of paper. The plastic board in corrugated or other laminated form offers many advantages over the conventional paper cardboard, including increased durability and especially resistance to water and chemicals as well as tear-strength.
In the manufacture of plastic board, such as laminated board made of synthetic themoplastic resin, it is desired to produce a high quality board with volume production techniques at a cost competitive with the more conventional forms of board. In the prior art the laminated plastic board has been fabricated by several techniques. One such technique is the extrusion from a single die of a plastic board having a pair of face sheets separated by longitudinally extending, laterally spaced thin walls which define parallel passages extending through the board. Such an extrusion technique is admirably suited for the production of boards which are especially suited for making cartons and the like. This technique, with the proper degree of control of temperatures and extrusion rate, results in a high quality board but the rate of production is not as great as desired. This extrusion process is disclosed in U.S. Pat. No. 3,274,315 granted Kawamura on Sept. 20, 1966.
Extruding and laminating techniques have been proposed in the prior art for making plastic board at a high rate of production. One such prior art process utilizes previously extruded plastic sheets, as supplied in rolls, and feeds one sheet through a pair of corrugating rolls to form a core sheet. A pair of face sheets are then supplied on opposite sides of the core sheet and bonded thereto. One difficulty with this process of making the corrugated board from preformed plastic sheets stems from the requirement for high temperature of the corrugating rolls. This causes the sheets to adhere to the corrugating rolls and results in deformation or undesirable stretching of the sheets. A process of this type is disclosed in U.S. Pat. No. 2,719,566 granted to Blatt on Oct. 4, 1955. A similar process is shown in U.S. Pat. No. 3,666,590 granted to Susuki et al. on May 30, 1972.
It has been proposed also in the prior art to achieve a higher rate of production of plastic board and to overcome some of the problems associated with the aforementioned processes by making laminated plastic board by corrugating a core sheet as it advances from the extruder and laying the face sheets on opposite sides thereof as they are advanced out of the respective extruders. This process has an advantage in handling the corrugated core sheet since the corrugating rolls may be at a lower temperature than the extruded sheet. For bonding the face sheets to the core sheet it has been proposed to use adhesive material at the ridges of the corrugations. It has also been proposed to use a melt-press bonding as disclosed in U.S. Pat. No. 3,723,222 granted to Kurita et al. on Mar. 27, 1973. It has also been proposed to achieve the bonding by maintaining the face sheets and the core sheet at sufficiently high temperatures to effect bonding at the time they are brought together.
The bonding of the face sheets to the core sheet by fusion and thus avoiding the need for adhesive material, is highly advantageous but it does pose certain control problems in maintaining the proper temperatures and rate of movement of the sheets. Even though these parameters are carefully controlled in making a board with a corrugated core sheet, the sheet tends to curl or warp after it is cooled to room temperature. Furthermore, it has been found that plastic boards having a corrugated core have a considerably lower bending strength along the direction of the corrugations than in the direction across the corrugations.